The present invention relates to thin film semiconductors, and in particular, to methods of making thin film semiconductors having a polymer substrate, and structures having a single crystal silicon film and polymer substrate.
There is a growing need for a variety of display devices, such as liquid crystal display (LCD), organic light emitting diode (OLED), and active matrix liquid crystal display (AMLCD) devices. These display devices are based on technology using thin-film semiconductor material. The thin-film semiconductor material is used to produce thin-film transistors (TFT), three-terminal devices in which an input signal controls the output current. TFTs are classified into those using polycrystalline silicon (poly-Si), amorphous silicon, or single crystal silicon (SCS) thin-film semiconductor material. TFTs perform switching and amplifying functions and can operate as a discrete device or as a building cell of integrated circuits. For example, an LCD device uses an array of TFTs to control the display process. In OLED devices, radiation is emitted as a result of electron-hole interactions in thin film organic semiconductors. An AMLCD device is a higher performance version of an LCD and uses an array of TFTs to control individual pixels.
TFTs made using amorphous silicon must be driven by standard integrated circuits. Amorphous silicon is a non-crystalline silicon possessing no long-range crystallographic order. As a result, amorphous silicon films possess poor electrical characteristics and inherently poor stability. Amorphous silicon film is suitable for producing transistors to switch individual pixels on and off, but is unable to handle logic and mixed signal functions needed for higher performance devices.
TFTs produced using poly-Si technology are superior to those made with amorphous silicon. Poly-Si is a crystalline silicon in which atoms are arranged following a specific pattern (long-range order) within limited grains. While this emerging technology is expected to eliminate the need for separate display drivers, required with TFTs made of amorphous silicon, and allow fabrication of drivers on poly-Si itself, and reduce the cost and size of display devices, poly-Si technology has been under development for several years and has yet to solve issues of poor uniformity and high cost of manufacture.
The use of SCS films creates superior electronic and photonic properties as compared to amorphous silicon and poly-Si films. SCS is a crystalline solid in which long-range order exists throughout the entire piece of material, significantly enhancing low-charge-carrier mobility. Also, the single crystal nature of the film provides better uniformity than amorphous silicon or poly-Si films.
Processes for transferring SCS film onto an oxidized substrate are known in the art. Exfoliation is one such technique. Exfoliation by the hydrogen ion implantation method generally involves implanting a silicon wafer with ions to create a defect structure in the wafer. Typically, the implantation ions are hydrogen ions. The ion-implanted side of the wafer is then brought into contact with an oxidized silicon substrate. The wafer and oxidized silicon substrate are heated to high temperatures, typically greater than 1,000° C. The heat treatment forms a bond between the ion-implanted side of the silicon wafer and the oxidized silicon substrate. The heat also causes the ion-implanted portion of the silicon wafer to separate from the silicon wafer.
Since processes for obtaining an SCS film on a substrate require relatively high temperatures, they are not suitable for polymer substrates. Consequently, most TFTs used in display devices are made using glass substrates. However, glass substrates result in significant weight of display devices due to the high density of glass employed therein. Also, glass substrates are rigid and possess a low tolerance for bending stresses. Moreover, glass is fragile and if dropped, TFTs made with glass substrates have a high risk of breakage.
Many applications require lightweight, flexible displays. There are many polymer materials that are much more flexible than glass and have less than about half the density of glass. As a result, some effort has been made to develop technology to replace glass substrates with polymer substrates. These technologies employ either amorphous silicon or poly-Si film transferred to a polymer substrate and are subject to the aforementioned drawbacks. Accordingly, there is a need in the art for devices employing SCS film on a polymer substrate.